6.5" vs 7" wheel - lap time difference

Hi!

I know this topic has been beaten to death all over the internet forums then revived and killed again, but I have a bit different and specific question. What will lap time difference be, if any, if I run 15x6.5 wheels instead of 15x7. I understand it depends on other variables, so here we go:

Car in question: '94 1.8 Miata with minimal power mods, around 10 hp more than stock.

Tyres: 195/50 15" Toyo R888 R-comp.

Tracks: a) ~0.75 mile long road course, with longest straights about ~1000 ft; my lap time is around 1:05 b) ~2-2.5 mile circuits, with lap times around 1:30-2:00

I'm asking because I have a set of very lightweight (~10 lbs) Sprint Hart wheels in 15x6.5 which I'm going to use on Mazda. However my wife runs 15x7 Advans (also pretty lightweight at ~12 lbs) on her daily Renault Clio RS and with a bit of persuading maybe I can swap the wheels between the cars. I'm just not sure there is a point in going to all the trouble of convincing her :)

So what do you think this half-inch is worth? 0.2 sec on 1 min lap? 0.1, 0.5?

Thanks!

• Dmitry

here's an excellent primer on tires ... it's mainly aimed at street tire classes, but if you scroll down to the bottom you'll get to a section headed optimal wheel size, which can give you some good info as to what size tires work best on what size wheels

http://www.facebook.com/notes/hollis-racing/street-touring-and-track-day-tire-faq-utqg-140/245008598916447

FYI...1 lb of rotational mass = 10 lbs of static weight

-8 lbs of wheel weight sould = -80 lbs of dead weight removed from the car.

It's easier if you look at wheel weight (and other unsprung weight) as a percentage of total car weight. Light cars are obviously more sensitive.

However, I think most of the actual instrumented tests show that wheel weight does not make a huge difference to lap times unless you're dealing with very large differences. It will make a difference to the responsiveness of the car and suspension compliance, although you may not feel the difference between a 10 lb and a 12 lb wheel unless you're driving back to back. A Miata on 12 lb wheels is pretty happy.

I doubt you'll see a huge difference in lap times. The 15x7 would likely be slightly faster, but that 195/50-15 will work pretty well on a 6.5".

I think it's been pretty well demonstrated for autocross that you should stuff the widest tire possible, on the appropriate sized wheel, that your car/class will allow. Considering the first track you listed sounds smaller than some autocrosses I've been to, I'd say widest wheel/tire combo possible for that track.

For the other track, I'd think light weight would win out over a 1/2" difference in rim width, but that's just speculation.

I'd guess around 0.25sec. The 15x6.5s would give a smoother ride and slightly increased MPG so you could use that as a bargaining chip with your wife.

But then there's the rotational mass issue...I think the best reason for you to run the 15x7s is for more even tire wear.

petegossett wrote: I think it's been pretty well demonstrated for autocross that you should stuff the widest tire possible, on the appropriate sized wheel, that your car/class will allow. Considering the first track you listed sounds smaller than some autocrosses I've been to, I'd say widest wheel/tire combo possible for *that track*.

I wouldn't say that, the track where my autocrosses are usually held is around a 1min lap (that's about what I got when my 'rolla was very nearly stock, minus the wheels & tires) but we often set up 1.5 minute courses using just part of the same track. It is a small track, probably better suited to go-karts than cars, but it's only different from most other tracks in that really short gearing helps.

Hi Dmitry,

Welcome to our board. You are probably the first visitor we've ever had from Latvia. You'll have to tell us more about the motorsport scene there. I used to enjoy watching Markko Martin from Estonia drive in the WRC. Many of us will be jealous that you have a Renault Clio, a car that has never been available in America.

Personally, I would use the 7" wide wheel. I think that the additional sidewall support will outweigh the weight gain.

Jerry

petegossett wrote: I think it's been pretty well demonstrated for autocross that you should stuff the widest tire possible, on the appropriate sized wheel, that your car/class will allow. Considering the first track you listed sounds smaller than some autocrosses I've been to, I'd say widest wheel/tire combo possible for *that track*. For the other track, I'd think light weight would win out over a 1/2" difference in rim width, but that's just speculation.

The link above with the tire article by Andy Hollis disagrees with the wider is better idea, at least with ST class tires.

Andy does mention that the idea of stuffing the largest tire you could get under the fenders came from Stock class runners on R-Comps ... the wheel width selection is much more critical when you get into ST tires .... the R888 is sorta an in between tire, not quite a Hoosier A6, but neither is it a Toyo R1R

What are the 2 wheels offset? Are they equal except for the 1/2"? (is 1/4" more on both inside, and outside of flange?)

All things being equal... TRULY equal except for the 1/2"... The 7" wheel may provide more consistent lap times for longer... the 6.5" rim may lead to more heat build up.

Thank you everyone for the input. Can't say my driving is good/consistent enough to really utilize estimated .2 sec gains, however the point of heat is more important. I suppose it will also somewhat impact tyre life. I think I will have a bit of discussion with my other half today.

Keith, re wheel weights I remember a couple of tests in GRM which showed that there wasn't measurable difference between 15 and 20 lbs wheels on a 3gen MX5. However I have a personal experience with replacing 18 lbs OEM Alfa Romeo wheels with 12 lbs Regamasters on my other half's Alfa (that was before we replaced it with Renault). Same tyres, same size (16x7.5), offset only 2 or 3 mm different. No real difference when on smooth surface, but on pot holes or stone roads (we have plenty of them in city center) contrast was bigger than when we've replaced worn shocks with new Koni yellows. I believe wheel weight is only really relevant when high damping frequencies are involved unless a wheel in question isn't really really heavy.

Jerry, oh yeah, Markko is a big name here in Baltics. And this also characterizes local motorsports: gravel rally is the biggest thing here, tarmac and circuit stuff, not so much. Reason being we still have loads of winding gravel roads. And if you like rallying, we had a European championship event recently, just check out youtube for "rally liepaja venstpils", there are loads of videos.

Re the Clio, I've just added it to my virtual garage. In contrast to usual Renaults RS models are not only a huge fun but also surprisingly nicely built.

mazdeuce wrote:

petegossett wrote: I think it's been pretty well demonstrated for autocross that you should stuff the widest tire possible, on the appropriate sized wheel, that your car/class will allow. Considering the first track you listed sounds smaller than some autocrosses I've been to, I'd say widest wheel/tire combo possible for *that track*. For the other track, I'd think light weight would win out over a 1/2" difference in rim width, but that's just speculation.

The link above with the tire article by Andy Hollis disagrees with the wider is better idea, at least with ST class tires.

There are a few unusual exceptions that are particularly notable in autocross, namely the 195/50-15 Toyo R1R. If you stuff that under a 15x7.5 rim (the widest allowed in STS), you can go quicker than you could with a 7 or 6.5 wheel. I'd expect a similar effect with an R-comp where the goal is the most tire you can run with the reasonable support of a wide wheel underneath. The local hotshoes that run Miatas tend to run the Toyo 195s on 15x7 at minimum.

On the other hand, Spec Miatas run 205/50-15 R1A and R888s (and other spec tires) on a 15x7 wheel slightly offset from stock (wider track). While that could be simply due to their restrictions forcing everyone to use the "appropriate tire/wheel size," I'm sure they put a good amount of thought into picking a sizing combo that would be highly reliable on track over long periods of time (where safety really matters!).

If you only do short doses of racing, I'd use the 195 15x7 combo. If you start racing for longer than 10 minutes a session, I'd think long and hard about switching back to 15x6.5 or upgrading your tire size.

kylini wrote: If you only do short doses of racing, I'd use the 195 15x7 combo. If you start racing for longer than 10 minutes a session, I'd think long and hard about switching back to 15x6.5 or upgrading your tire size.

I thought the opposite was true, i.e. 6.5 wheel putting more heat in the tire which isn't desirable for long runs? Competition runs will be short, around 3-4 laps at a time, but while training I usually drive for 20-30 minutes between breaks.

moro wrote:

kylini wrote: If you only do short doses of racing, I'd use the 195 15x7 combo. If you start racing for longer than 10 minutes a session, I'd think long and hard about switching back to 15x6.5 or upgrading your tire size.

I thought the opposite was true, i.e. 6.5 wheel putting more heat in the tire which isn't desirable for long runs? Competition runs will be short, around 3-4 laps at a time, but while training I usually drive for 20-30 minutes between breaks.

I'm not an expert nor a pro-racer. That's just how I interpreted two anecdotes. Listen to other people first! Better yet, test it out.

At the time the Spec Miata specifications were laid out, there were very very few options for wheels in 15x7.5 and 15x8. That's actually still the case. 15x7, on the other hand, is very popular. So it's an obvious choice for a budget series.